Proto, Scoru: RPC to retrieve stakers per commitment

Context

#4235 (closed)

• Add a RPC to retrieve stakers per commitment for a given rollup.
• Add an according tezt test to ensure the proper retrieval of informations.

Manually testing the MR

To trigger the according tezt test:

dune exec tezt/tests/main.exe -- --file sc_rollup.ml "stakers_per_commitment"

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR